Cell division orientation must be tightly controlled for the generation of normal tissue architecture. In the skin, asymmetric cell divisions that are driven by oriented mitotic spindles promote both the stratification and differentiation of the embryonic epidermis. While some of the proteins required for spindle orientation have been identified, we do not fully understand how this molecular machinery generates forces on astral microtubules to allow precise control of cell division orientation. In addition, the essential requirement of spindle orientation in embryonic epidermal development has prevented analysis of the function of these divisions in adult skin. Roles for spindle orientation in the morphogenesis of hair follicles, which are highly organized epidermal appendages, have not been addressed. Similarly, whether spindle orientation is required for homeostasis of the adult interfollicular epidermis has not been tested. This is despite long- standing hypotheses that loss of spindle orientation could promote tumorigenesis. Our preliminary studies have identified an unexpected mechanism required for spindle orientation in keratinocytes which we will elucidate in detail in Aim 1. These mechanistic studies allowed us to generate a mutant mouse model in which spindle orientation was randomized in the epidermis, resulting in neonatal lethality. Importantly, disruption of spindle orientation in adult animals caused both severe hair follicle defects and interfollicular hyperproliferation and local invasion. We will use this new genetic model to determine the specific roles for spindle orientation in hair follicle morphogenesis (Aim 2) and adult interfollicular epidermal homeostasis and tumorigenesis (Aim 3). Together, these data will provide a deeper mechanistic understanding of spindle orientation which has essential functions in epidermal, cardiac and neural development as well as the adaptive immune response. In addition this work has direct relevance for both understanding and eventually treating alopecias/hair follicle disorders and skin cancer.